The field of the present invention is the construction and use of power amplifiers. More particularly, the present invention relates to power amplifiers that exhibit insensitivity to antenna loading with increased efficiency when operable in multiple power modes.
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and/or data communications in a handheld, telephone-like communication transceiver, a wireless personal digital assistant (PDA) or computer interface. While the different modulation and transmission schemes each have advantages and disadvantages, common goals for all of these devices is minimizing the size and cost of the handset, while maximizing the performance of the transceiver.
In a wireless device an RF power amplifier accepts the RF signal from transceivers and amplifies this RF signal for wireless transmission via the antenna connected to the output of RF power amplifier. In some communication systems, the power amplifier can improve its performance by operating in more than one power mode. By using multiple power modes, mobile communication devices with a limited power source may operate longer or with smaller batteries. Unfortunately, constructing a power amplifier to operate in multiple power modes typically requires adding more components, or in the alternative, making design tradeoffs that allow one power mode to operate at good efficiency while sacrificing efficiency, linearity, or other figure of merits in another mode.
Conventional RF power amplifiers may include multiple stages and, in some applications, include multiple amplification paths. In some configurations this multiple path arrangement is referred to as a “balanced amplifier.” Regardless of the architecture, conventional balanced power amplifiers require coupling circuitry to combine the outputs of multiple paths together. Further, because the load presented to the power amplifier changes with variations in the antenna input impedance, conventional RF power amplifiers, whether those that use a single or dual path amplification scheme, generally employ one or more isolators at the output of the amplifier. Antenna impedance changes frequently depending on the location, surrounding environment, and operating mode of the mobile device. For example, the impedance of the antenna may vary between, for example, 20 and 150 ohms, depending on the operating environment of the mobile device. Without a protective isolator or other special protective circuitry, this variation in the impedance presented to the power amplifier causes the performance of the power amplifier to vary, thus degrading overall performance of the mobile device. The isolator minimizes the impedance variation presented to the output stage of the power amplifier. Unfortunately, the isolator is a relatively large and costly component that, because of the possibility of physical contamination, is difficult to integrate onto the same structure on which the power amplifier is fabricated.
Further complicating the design of power amplifier systems, some current and evolving telecommunications standards require operation at more than one power level. Since each power level is typically implemented using a different power chain, many additional components are used to provide for switching and impedance matching. These additional components take up valuable design space, as well as consume power, which is a limited resource on most portable wireless devices.
In constructing a balanced amplifier to operate with multiple power modes, for example, a high power mode and a low power mode, additional mode switching control modules are added to the balance power amplifier architecture. Even with this additional part-count, engineering tradeoffs are typically made so that the high power mode operates quite efficiently, but the low power mode is forced to operate at a less desirable efficiency level. Accordingly, known multi-mode balanced amplifiers are bulky, cumbersome, and do not operate with good efficiency in the lower power modes.
U.S. Pat. No. 6,954,623, issued Oct. 11, 2005, and entitled “Load Variation Tolerant Radio Frequency (RF) Amplifier”, discloses an amplifier construction that provides for improved isolation over previously known balanced amplifiers. However, advancing telecommunications standards, as well as the desire to make mobile devices smaller and more powerful, places further burdens on the design and construction of power amplifiers. Accordingly, there exists a need for a power amplifier system with improved efficiency, sufficient linearity, and with a construction that can be compactly integrated into wireless devices. In particular, there exists a need for a power amplifier that can efficiently operate in its low power mode, without the need to excessively increase part count and sacrifice much performance of other figures of merit.